The production of fuel ethanol or other products from lignocellulosic feedstocks provides an attractive alternative to the fuel ethanol feedstocks used to date, such as corn, sugar cane and sugar beets. Cellulose is the most abundant natural polymer, comprising, on average, about one third of all plant matter. While many lignocellulosic materials, such as wood and cotton, are in high demand, other types have limited market value. These feedstocks are therefore potentially inexpensive and, as such, there is an untapped potential for their use as a source of ethanol. The lignocellulosic feedstocks that are the most promising for ethanol production include agricultural wastes, grasses, forestry wastes and sugar processing residues.
A series of chemical and biological treatments are commonly employed to convert the long chain polysaccharide sugars contained in the feedstock to shorter chain oligomers or monomers, which, in turn, are fermented to ethanol or other fermentation products. Such treatment stages may involve two primary processes, commonly referred to as pretreatment and enzymatic hydrolysis. The pretreatment may be effected by chemical addition, including acid, alkali, oxidizing agents or organic solvents, although hydrothermal pretreatments are known that employ hot water. If an acid pretreatment process is utilized, the feedstock may be subjected to steam and acid at a temperature, acid concentration, and length of time that are sufficient to hydrolyse a small portion of the cellulose to glucose and to hydrolyse the hemicellulose to xylose, arabinose, and other sugars monomers or oligomers, depending on the constituents of the hemicellulose.
Subsequent to an acid pretreatment reaction, the pretreated feedstock is quenched by the addition of a base, such as sodium hydroxide, ammonia or other base, and by lowering the stream temperature. This action also conditions the feedstock for the subsequent enzymatic hydrolysis stage. However, as a result of base addition (or acid addition if alkali pretreatment is employed), inorganic salts are produced, for example sulfate salts. This adds to the existing salt load resulting from the significant amounts of inorganic salts present in the feedstock itself and which are carried through to this stage of the process. The resultant high levels of salts originating from these sources can be problematic in downstream unit operations, as discussed below. Subsequent to pretreatment, the cellulose remaining in the pretreated feedstock is converted to monomeric glucose by the use of an enzymatic mixture, primarily composed of cellulase enzymes.
After the completion of the pretreatment and enzyme hydrolysis stages, the process stream is composed of a liquid phase containing the fermentable sugars and other water soluble compounds, including the salts that are carried through to this stream. The process stream also typically contains an insoluble solids phase, comprised of lignin and other insoluble components. Lignin is a bio-polymer that differs from cellulose and hemicellulose in that it is not composed of carbohydrates and therefore is not a source of fermentable sugar. The insoluble lignin rich fraction that remains may either be filtered out of the stream after enzymatic hydrolysis, or may be carried forward through subsequent processing steps. Regardless of the point at which the lignin is separated from the stream, the lignin rich solids may be used as a fuel material for a boiler system, which typically produces steam for either use in the process, including heat that may be used in the process, electricity production, building heating, or a combination thereof.
The stream leaving the enzyme hydrolysis stage, either with or without the insoluble lignin, is fed to a fermentation stage, which employs any of several microbial organisms to convert the glucose, xylose, arabinose, or any combination of these, into ethanol or other fermentation products. This stream is typically called fermentation beer. The fermentation beer may be fed to a distillation system that strips the ethanol out of the solution and the ethanol is then further processed as required for sale as a transportation fuel.
The residual liquid phase remaining after removal of alcohol by distillation or other means is typically called still bottoms, and contains a complex mixture of organic and inorganic components, derived from the process chemicals, biological treatment stages, and from the feedstock. For example, the still bottoms stream will generally contain large quantities of sulfate, or other conjugate base of the acid used in the pretreatment step, the cation of the base used for neutralization at the completion of pretreatment, various protein components, polyols, soluble lignin and lignin derived compounds, unfermented sugars, potassium salts, magnesium salts, chloride salts, and other trace inorganic species.
In the corn ethanol industry, the still bottoms stream is typically dried and blended with the residual grain material and sold as an animal feed. However, the composition of the still bottoms from the cellulose ethanol process may not be suitable for this market. Consequently, a disposal method that has been suggested for still bottoms derived from cellulosic conversion processes is biological waste water treatment. While biological waste water treatment of such still bottoms streams removes the organic constituents, many of the inorganic components either present processing difficulties, or pass through the system unaffected. The final effluent then contains a large quantity of dissolved salts which may not meet effluent discharge criteria, or may pose processing problems if the waste water effluent stream is recycled.
A known waste disposal technique is incineration, which allows the recovery of heat from the combustion of organics. However, the incineration of a still bottoms stream derived from cellulosic conversion processes is particularly challenging due to the presence of inorganic components such as sulfate, ammonium, potassium, chloride, and other elements. In particular, the sulfate will convert at least in part to sulfur oxides, and the ammonia will convert at least in part to nitrogen oxides, both of which are regulated pollutants, and will require additional equipment to control and reduce them to acceptable levels. Moreover, the potassium and chloride may cause a low melting point slag and possibly volatile fume that will foul components of the system and generate dust. While technically feasible, incineration of the still bottoms stream derived from cellulosic conversion processes presents several challenges that greatly increase the cost and complexity.